A. Mbaruku

High Field Superconducting Solenoids Via High Temperature Superconductors

High-field superconducting solenoids have proven themselves to be of great value to scientific research in a number of fields, including chemistry, physics and biology. Present-day magnets take advantage of the high-field properties of Nb3Sn, but the high-field limits of this conductor are nearly reached and so a new conductor and magnet technology is necessary for superconducting magnets beyond 25 T. Twenty years after the initial discovery of superconductivity at high temperatures in complex oxides, a number of high temperature superconductor (HTS) based conductors are available in sufficient lengths to develop high-field superconducting magnets. In this paper, present day HTS conductor and magnet technologies are discussed. HTS conductors have demonstrated the ability to carry very large critical current densities at magnetic fields of 45 T, and two insert coil demonstrations have surpassed the 25 T barrier. There are, however, many challenges to the implementation of HTS conductors in high-field magnets, including coil manufacturing, electromechanical behavior and quench protection. These issues are discussed and a view to the future is provided.

Relationships Between Conductor Damage, Quenching and Electromechanical Behavior in YBCO Coated Conductors

The implementation of emerging superconducting materials into magnet systems with long service lifetimes requires a thorough understanding of their engineering properties, including their quench and electromechanical behaviors. Furthermore, it is essential to understand the role of defects in the conductor, whether they be pre-existing defects from the conductor manufacturing process that locally reduce Jc, or local defects that result from a non-destructive quench (i.e., a quench that may reduce Jc locally but does not significantly affect the end-to-end behavior). This paper reports results on both of these types of defects and the interplay between quenching and electromechanical behavior. Quench studies investigate the initiation and propagation of quenches in coated conductors. Disturbances in homogeneous conductors are initiated by a pulsed heater attached to the conductor. Disturbances in locally damaged conductors are initiated by increasing the transport current above the Ic at the local defect but below the end-to-end Ic. Samples are quenched to determine the minimum quench energy and the quench propagation velocity. Homogeneous samples are also quenched to the point of initiating local damage, thereby identifying the maximum allowable hot-spot temperature or hot-spot temperature gradient. Samples used in quench studies are subsequently used in Ic-strain measurements to determine how quenching affects subsequent performance. Samples that exhibit reduced Ic from quenching, and samples from regions adjacent to such damaged samples, are studied. It is found that quenching can reduce the electromechanical performance of conductors that do not initially show a reduction in their electrical performance.

Mechanical properties of non-functionalized multiwall nanotube reinforced polycarbonate at 77 K

Mechanical properties of non-functionalized, multiwall carbon nanotube (CNT) reinforced polycarbonate composites are studied at room temperature and 77 K. Five sample groups are tested, ranging from 0 to 10.0 wt% CNT. The dispersion, interfacial bonding, bundling and CNT content, as well as the testing temperature, play a major role as regards mechanical properties. Mechanical testing shows increase in strength with increasing CNT content as well as an increase in Youngs modulus and a decrease in ductility. The distribution of yield strength data for each sample group is analyzed using Weibull distributions. It is evident that interfacial debonding increases at low temperature. Higher CNT concentration samples are affected the most, which is reflected in a decrease in their impact on the mechanical properties at 77 K compared to RT. Scanning electron microscopy of fracture surfaces supports the interpretation of the measurement results.

Development of a 5 T HTS insert magnet as part of 25 T class magnets

The development of a 25 T superconducting magnet is usually envisioned with the use of an HTS insert coil. Previously, we reported the successful development of a 3 T coil in a 19 T background field based on BSCCO 2212 conductor. Here we report on the progress toward a larger 5 T insert with 38 mm free bore. The design is introduced, which calls for 2 stacks of double pancakes and an outer layer wound section, all electrically in series. Reacted conductor is used with insulated steel tapes as reinforcement. Results in terms of field dependence of the critical currents and stress tolerance are presented for both bare conductors and double pancakes. The latter are tested in a 19 T, 0.17 m cold bore, magnet assembly.

Fatigue Behavior of Y–Ba–Cu–O/Hastelloy-C Coated Conductor at 77 K

Superconducting materials are subjected to various loading in motors, transformers, generators, and other magnet applications. The loading conditions include bending, tension, compression, and fatigue, and result from coil manufacturing, thermal cycling, quenching, and normal operation. Each of these loading conditions can affect the performance of the superconductor and thus the magnet and system. It is important, therefore, to understand the electromechanical behavior of the superconducting material to optimize the design. Here we report the effects of mechanical fatigue at 77 K on the electrical transport properties of YBa2 Cu 3O7-delta/Hastelloy-C coated conductors. The effects of longitudinal tensile fatigue on the critical current and the n-value are reported. Strain controlled fatigue studies include strains up to 0.495% and strain ratios of 0.2 and 0.5. Scanning electron micrographs of the fatigued conductors are used to identify the sources of failure. Crack formation is believed to be the cause of I c degradation in fatigued samples. Further, the fatigue strength and ductility behaviors analyzed using a 5% reduction in I c as the electrical definition of failure showed that the fatigue strength exponent is within the values found for metals but both the fatigue ductility coefficient and exponent show that the material tested is brittle.

Effect of processing defects on stress-strain-I/sub c/ for AgMg sheathed Bi-2212 tapes

Manufacture of multifilamentary BSCCO tapes involves rolling of billets to final form. In BSCCO-2212 tapes, one method used to control the microstructure and increase J/sub c/ is intermediate annealing during the deformation process. At times, small bubbles have been observed on the tapes after annealing. Although such bubbles do not remain at the end of fabrication, the question of possible effect on superconducting performance has been studied. The sections that had bubbles after annealing were marked and the fabrication process completed on the tape. After final reaction of the tape, comparisons of stress-strain-I/sub c/ for bubbled and nonbubbled sections were performed. Here we present electrical, mechanical and microstructural studies on bubbled and bubble-free sections of tape. It was found that sections with bubbles had lower I/sub c/ than sections with no bubbles, though both had similar onset of I/sub c/ degradation at /spl sim/0.40%.

Development of a low-temperature electro-mechanical testing device

Understanding the strain-sensitivity of high temperature superconductors is important for the development of applications. Conductors for magnets experience mechanical loads during all stages of manufacturing and thermal cycling, as well as Lorentz force induced loads during operation. Thus, it is important to study the effects of mechanical loads on HTS conductors in the presence of magnetic field. Here we report on the development of a tensile testing device that was designed to characterize the in-field electromechanical behavior of HTS conductors. The device is capable of applying tension or compression, controlled fatigue cycles, and in-situ transport critical current measurements. We report on the development and capabilities of the device, as well as the initial stress-strain results at room temperature.

Electro‐Mechanical Behavior of YBCO Coated Conductor in Tension

Recently, high critical current densities in long‐lengths of YBa2Cu3O7−x coated superconductors have been obtained. Since in magnet applications, stresses are generated during coil construction and operation and the fact that the superconductors are brittle and sensitive to cracking, understanding the effects of such stresses and strains on the electrical performance is very critical. Here, we report on the mechanical properties of such coated conductors under axial tension, including the dependence of critical current density upon strain. Microstructural analysis by Environmental Scanning Electron Microscope relates the superconducting performance to the tensile conditions to which the conductor is subjected.

Development of a LNe test facility

To characterize the electrical and mechanical properties of HTS-conductors at elevated temperatures, a Ne-liquefaction system and cryostat has been developed and built. The device makes use of a coil type condenser inside the experiment dewar. The condenser is fed by LHe. Ne-gas, which enters the dewar at room temperature, liquefies on the coil windings and drips to the bottom of the dewar, where the experimental setup is located. Used He-gas and Ne-gas are recovered. Approximately 1.5 l of LNe was condensed into the dewar in about 1 h after precooling the outer jacket of the dewar with LN2. The device has been used to measure zero-field and in-field transport properties of Bi-2223 and YBCO high-temperature superconductors.

Mechanical Properties of Carbon Nanotube Reinforced Polycarbonate at Cryogenic Temperature

Carbon nanotube (CNT) reinforced polymers are of high interest for various industries due to their unique mechanical and electrical properties. Most research has been done at room temperature (RT), but little is known about properties at cryogenic temperature. This paper presents results on CNT-polycarbonate (PC) composites with respect to mechanical properties at 77 K in comparison with RT. CNT-PC composites with 0wt% (neat), 0.1wt%, and 1.0wt% CNTs have been studied. Results imply that the CNT effects are more obvious at low temperature and are seen in the form of serrations in the stress-strain plot. No significant difference has been noticed between the neat and reinforced samples at either temperature. However, it was determined that the strength increases drastically while the elongation decreases at low temperature as compared to RT. SEM images confirm that the samples at low temperature exhibit brittle failure. Additionally, it can be seen that at low temperature the nanotubes align with the direction of tensile force while the nanotubes at RT align with the sample surface.